1. Field of the Invention
This invention relates to a feedback control system, and more particularly, to a feedback system for controlling the position of tilting burners of a coal-fired furnace.
2. Description of Related Art
In the operation of present-day steam generating units, it is common to physically raise or lower the burners over a considerable distance to make use of more or less heat absorption surface in the furnace and thereby effect a wide range control of the temperature of the combustion gases leaving the furnace. Thus, burner tilt-drive devices have been developed to facilitate movement of the burners at each corner of the furnace. Conventionally, such devices include pneumatic cylinders which manipulate rotary actuators to control the position of the burners. The pneumatic cylinders include a piston which divides a cylindrical housing into first and second chambers on opposite sides of the piston. To adjust the burner, pneumatic fluid is supplied to or vented from the first and second chambers.
The Clean Air Act Amendment places an emphasis on low NOx emission, thus necessitating tighter control of burner tilt to improve combustion. Conventionally, control of the burner tilt corners had been done collectively, to reduce cost. However, with the need for tighter burner control, it has been proposed to isolate each burner corner and provide individual control thereof. Further, to ensure optimum combustion, feedback position for each burner tilt device at each burner corner has been proposed. However, to establish such feedback, the rotational position of the final control element must be monitored, requiring that the feedback device be exposed to the excessive heat of the furnace. Such devices may thus be exposed to temperatures exceeding 240.degree. F. consequently, feedback devices tend to degrade with excessive heat and ultimately induce errors in the positioning of the burners tilt devices. In addition, conventional control systems have included linkages which may induce further errors. Even further, individual control of burner corners utilizing conventional feedback control systems generally requires high cost modifications to existing systems.